1. Field of the Invention
The present invention relates to digital film processing methods. More particularly, the present invention provides a digital film processing method wherein a dye is formed in the photographic element during processing in order to provide an increased signal range.
2. Description of Related Art
In traditional film photography, the photographic film includes one or more layers of a photosensitive material (typically silver halide). When a picture is taken, the light from the scene interacts with the film's photosensitive material to produce a chemical change in the photosensitive material. This chemical change is in direct proportion to the intensity of the light. The greater the intensity of light from the scene, the greater the chemical change in the photosensitive material. As described in greater detail below, the photographic film is then chemically developed in order to produce an image based on the chemical change.
Conventional black and white photographic film generally has a single layer of silver halide emulsion coated on a transparent film support. Color photographic film generally includes multiple layers of silver halide in combination with dye forming coupling agents. Each silver halide layer in color photographic film is sensitive to a different portion of the visible spectrum. Typically, color film includes one or more silver halide layers sensitized to each of blue, green and red portions of the visible spectrum, and the coupler in each layer is capable of forming a dye of a color which is complimentary to the color of light to which the layer is sensitized. For example, a silver halide layer which is sensitized to blue light will include a coupler associated with the formation of a yellow dye.
In traditional chemical development processes, the exposed film is developed using a developing agent. The developing agent chemically reduces the exposed silver halide to elemental silver. The amount of elemental silver produced in any given area of the film corresponds to the intensity of light which exposed that area. Those areas of the silver halide where the light intensity was the greatest will have the greatest amount of elemental silver produced. In contrast, in those areas of the silver halide where the light intensity was low, a very small amount of elemental silver is produced. The pattern of elemental silver thus forms an image in the silver halide layers.
During the traditional chemical development process, the highlight areas of the image (e.g., areas of the film which were exposed to the greatest intensity of light) will develop before those areas of the film which were exposed to a lower intensity of light (such as areas of the film corresponding to shadows in the original scene). A longer development time allows shadows and other areas of the film which were exposed to a low intensity of light to be more fully developed, thereby providing more detail in these areas. However, a longer development time will also reduce details and other features in the highlight areas of the image. Thus, the development time in a traditional chemical development process is typically chosen as a compromise between highlight details, shadow details and other features of the image which are dependant upon the duration of development. After development, in the case of black and white photographic film, the image is fixed by dissolving the undeveloped silver halide. The developed negative can then be used to produce a corresponding positive image on photographic paper by methods well known to those skilled in the art.
In the case of conventional color photographic film development, elemental silver is formed in the silver halide layers as described above. After the developing agent has reduced the exposed silver halide to elemental silver, the oxidized developing agent reacts with the couplers in the film to produce dye clouds around the grains of elemental silver in each of the layers. The color of the dye clouds in each layer of the film is complementary to the color of light the layer has been sensitized to. For example, the red sensitive layer typically produces cyan dye clouds, the green layer produces magenta dye clouds, and the blue layer produces yellow dye clouds. At this point, each layer of the color film includes both a silver image and a dye cloud image. The elemental silver and undeveloped silver halide are then removed from the film by bleaching and fixing, leaving only a dye image in each layer of the film. Since the dye in each emulsion layer is formed in an imagewise manner, the developed film will generally have yellow, magenta and cyan colored negative images in the blue, green, and red-sensitive emulsion layers, respectively. The color negative can then be used to produce a corresponding positive image on photographic paper by methods known to those skilled in the art.
The negative, or the corresponding positive image, can also be digitized using a conventional electronic scanner to produce a digital representation of the image. Scanning of negative images on film is typically accomplished by passing visible light through the developed negative. Light transmitted through the film is attenuated by developed silver (black and white film) or by the dye layers (color film), thereby allowing one to capture and record a digital representation of the image. The transmitted light is then passed through filters having appropriate spectral sensitivities such that the densities of the yellow, magenta and cyan dyes may be detected for each location on the film. The density values detected in this way are indirect measures of the blue, green and red light that initially exposed each location on the film These measured density values constitute three values used as the blue, green and red values for each corresponding location, or pixel, in the digital image. Further processing of these pixel values is often performed to produce a digital image that accurately reproduces the original scene and is pleasing to the human eye.
A relatively new process for developing film is digital film processing (“DFP”). Digital film processing digitizes, i.e., electronically scans, the silver image during the development process. The elemental silver image developed in each of the layers of the photographic film is used to construct a digital image of the scene photographed. The developing film is scanned with infrared (“IR”) light so that the scanning light will not fog the film. The image can be scanned at different times during the development process in order to acquire additional information from the photographic film. The digitized images are then electronically processed to determine the colors associated with each location. The resulting digital image can then be printed or manipulated, as desired.
As discussed previously, conventional color negative film includes dye precursors (specifically, couplers) in the light sensitive silver halide emulsion layers, and these couplers react with oxidized developing agent to form dyes in an imagewise manner. However, at least three different couplers must be used in at least three different emulsion layers of the film in order to allow for the formation of differently hued images which correspond to the different spectral sensitivities of each emulsion layer. For example, the emulsion layer(s) sensitized to blue light include a coupler which forms a yellow dye during development, while the emulsion layer(s) sensitized to green light include a coupler which forms a magenta dye. The dyes formed upon development of conventional color negative film do not attenuate IR light. Therefore, when conventional color negative film is subjected to DFP using IR light, the dyes formed during development will not attenuate the IR light used during scanning. Only the developed silver in the emulsion layers will attenuate the IR scanning light.